US Day 3-7 Hazards Outlook
NWS Weather Prediction Center College Park MD
239 PM EST Wed Dec 04 2019
 
Valid Saturday December 07 2019 - Wednesday December 11 2019
 
Hazards:
- Heavy precipitation across portions of California, the Central Great Basin, the Pacific 
Northwest, and the Southwest, Sat, Dec 7.
- Heavy rain across portions of the Northeast, the Tennessee Valley, the Mid-Atlantic, the Southern 
Appalachians, the Southeast, and the Ohio Valley, Mon-Tue, Dec 9-Dec 10.
- Flooding occurring or imminent across portions of the Northern Plains.
- Much below normal temperatures across portions of the Great Lakes, the Upper Mississippi Valley, 
and the Northern Plains, Mon-Wed, Dec 9-Dec 11.
- Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Sat-Mon, Dec 
7-Dec 9.
- High significant wave heights for coastal portions of mainland Alaska, Tue, Dec 10.
 
Detailed Summary:

Much of the lower 48 will not see significant weather-related hazards to start off the Day 3-7 
period (Dec 7-11), but a major amplification of the jet stream may bring heavy precipitation and a 
shot of Arctic air to the central and eastern states early next week.

Starting in the West, a strong storm in the eastern Pacific will move steadily eastward through the 
end of the week and reach the West Coast Saturday. The current guidance is very consistent with 
yesterday's forecast showing the heaviest precipitation falling Friday evening through Saturday 
from southwestern Oregon to the San Francisco Bay area. Snow is likely across the higher terrain in 
the Sierra Nevada and Shasta ranges, where over a foot is likely. Elsewhere across this region, 
rainfall amounts will generally average in the 1-3 inch range.

After a relatively quiet Sunday, the amplifying upper pattern will strengthen a cold front over the 
Plains states Monday, which will steadily progress eastward through midweek. Out ahead of it, moist 
and mild air will stream up from the south and bring the threat for heavy precipitation to the 
eastern U.S. Monday through Tuesday. One area, associated with a retreating warm front, is expected 
over eastern sections of New England during Monday. At this time, temperatures appear warm enough 
that much of the precipitation will be rain, but some snow or freezing rain at the start is 
possible where cold air is trapped at the surface. Then on Tuesday, as the cold front advances 
eastward, there's another threat for moderate to locally heavy rain across much of the same area. 
Rainfall for the 2-day period will generally be around 1-1.5 inches. Farther south, showers and 
scattered thundershowers will develop ahead of the cold front where deeper Gulf of Mexico moisture 
resides. Right now, the best threat appears to be over the Tennessee Valley and southern 
Appalachians, where 1-2 inch rains are possible Monday-Tuesday. However, there's a lot of 
uncertainty among the guidance with developing low pressure along the front, which is resulting in 
differences in its forward speed. So confidence right now is relatively low with this latter area.

Behind the front, Arctic air will spill south and eastward through the Plains Monday, Midwest/Ohio 
Valley Tuesday and eventually through the mid-Atlantic and Northeast Tuesday night and Wednesday. 
At the moment, the most anomalously cold readings are likely over the northern Plains and Upper 
Mississippi Valley, where highs will struggle to get above zero.

In Alaska, strong southerly flow ahead of a north to south oriented front will tap moisture from 
the tropics and bring the threat for very heavy precipitation to sections of the south-central and 
southeastern mainland this weekend into early next week. The main threat will be across the Chugach 
mountains, Kenai Peninsula and Alaska range this weekend, with heavy snow across higher elevations 
and rainfall of potentially several inches per day at lower elevations. Precipitation is expected 
to shift eastward early next week, with moderate to heavy amounts reaching the far southeastern 
mainland and into the northern Panhandle region. Amounts will likely be lower as the connection to 
the tropics is cut off. In addition to the rains, there's a chance for high waves late 
Monday-Tuesday in the Gulf of Alaska from the Kenai Peninsula to the Panhandle.

Klein